1. Field of the Invention
The present invention in this specification relates to a light-emitting display device in which an oxide semiconductor is used and a method for manufacturing the light-emitting display device.
2. Description of the Related Art
It has been promoted to develop light-emitting elements where electrons are injected from one electrode and holes are injected from the other electrode into a stacked body containing an organic compound sandwiched between the pair of electrodes and the electrons and holes are recombined in the stacked body so that a light-emitting material in the stacked body is excited and thus light emission can be obtained. Further, development of light-emitting devices and display devices using the light-emitting elements has been also promoted.
Being self-light-emitting type ones, such light-emitting elements are superior in visibility with low dependence on a viewing angle, and thin shape and weight saving can be realized easily. Therefore, such light-emitting elements are expected to be applied to the use of flat panel displays of the next generation. In addition, it is also possible to manufacture such elements in a flexible film such as plastic, the elements are expected to be used for mobile displays.
A light-emitting display device using this light-emitting element can be roughly divided into two types, that is, a passive matrix type and an active matrix type. In the active matrix light-emitting display device, a thin film transistor (TFT) is electrically connected to each pixel to control light emission of a light-emitting element.
Thus far, an inorganic semiconductor material typified by silicon has been widely used for a TFT of the active matrix light-emitting display device. However, high temperature treatment is needed in order to form the inorganic semiconductor material typified by silicon as a semiconductor layer; therefore, it is difficult to use a flexible material such as plastic or a film as a substrate.
On the other hand, a TFT in which an organic semiconductor material is used as a semiconductor layer can be formed even at relatively low temperature; therefore, it is possible to manufacture in principle a TFT not only over a glass substrate but also over a substrate having low heat resistance such as plastic.
However, TFTs using organic semiconductor materials as semiconductor layers have disadvantages of low reliability and low mobility (see Patent Document 1).
On the other hand, TFTs whose active layers are formed using oxide semiconductors have high electric field mobility. Oxide semiconductor films can be formed at a temperature of 300° C. or lower by a sputtering method or the like, and a manufacturing process of the oxide semiconductor films is easy.
For example, a technique is known, by which a TFT is formed using zinc oxide or an In—Ga—Zn—O-based oxide semiconductor as an oxide semiconductor film and such a transistor is used as a switching element or the like of an image display device (see Patent Documents 2 and 3).